Study: more testing essential to defeat bovine TB

A new study on the spread of bovine TB shows that only with more frequent testing of cattle will the disease be defeated in England, writes Oliver Tickell. It has already worked in Scotland, and is on the way to doing in Wales - where some herds are tested every six months.

The researchers have shown without doubt that killing badgers will have little effect, whilst employing the policies of Wales and Scotland, where badgers are not culled, will continue to have a dramatic impact on reducing TB in cattle.

A new study by scientists from Queen Mary University shows that the most effective way to eliminate TB from the UK's cattle herds is to test them more often.

The main reason is that current TB tests fail to detect many cattle that are incubating the disease. Infected cattle that are not identified will then go on to infect other members of the herd.

"The main conclusion of the analysis conducted here is that more frequent testing is leading to lower TB infections in cattle both in terms of TB prevalence as well as TB incidence", the paper concludes.

Lead author Dr Aristides Moustakas, said: "It is clear that the Welsh policy of frequent testing up to every six months and the Scottish policy of risk-based surveillance are producing reductions in the both the incidence and prevalence of TB in cattle."

And his co-author Professor Matthew Evans, concurred: "It is clear that testing cattle frequently is the most effective way of reducing Bovine TB. Farmers and policymakers should not ignore this evidence which is based on the government's data."

The paper does not address the effectiveness of badger culling, as currently pursued in three English counties, as a means of controlling bovine TB. But the implication clear - that it is at best a strategy of secondary importance.

Success in Scotland and Wales, failure in England

In the study, the scientists compared the success at tackling TB of England, Wales and Scotland, since "regional differences in TB detections may provide insights of different policies against eradicating the disease."

Scotland has had a risk based surveillance testing policy under which high risk herds are tested frequently, and in September 2009 was officially declared as TB free.

Wales has had an annual (or more frequent) testing policy since January 2010, and in some Welsh counties cattle are tested every six months. Under this regime bovine TB in cattle has undergone a sharp decline: "Both the number of new herd incidents and number of herds not TB free are declining in Wales", states the report.

But in in England several herds are still tested every four years except some high TB prevalence areas where annual testing is applied. The sometimes long period between tests in England gives infected cattle that were missed abundant opportunity to infect other herd members.

As the authors point out, the "overall increase pattern of both TB incidence and prevalence in cattle in GB is thus driven by the English regions. Scotland and Wales both have a declining number of new herd incidents as well as herds not TB free and thus the current programme applied, all else being equal, appears to be leading to eradication or control of the disease ...

"The main result derived here from statistical analysis of publicly available data from the British Government show that increased cattle testing leads to TB control or possibly eradication as exemplified by the results in Wales. This conclusion fully supports the outputs of a computational model suggesting that all eradication scenarios included cattle testing frequencies of annual or even more frequent testing."

More TB infections detected in winter

The study also examines the fact that most 'new herd' infections are detected in winter, and that most 'new cattle' infections in non TB-free herds are detected in late winter.

One reason for the winter detection spike is that that's when most TB tests are carried out - suggesting that infections are being missed at other times of year: "This implies that the more one tests, the more infected cattle is likely to detect and thus should test more often."

Another explanation is that the TB spreads more easily from animal to animal in winter housing quarters when cattle are kept in close proximity.

And a further important point emerges: "new herd incidents as well as total tests on herds are lowest during summer months, when cattle are out in the field, the period that interactions with badgers are maximised. If badgers are the agent of cattle infection it is logical to test cattle during summer months."

Indeed it appears rather extraordinary that, given many farmers' insistence that badgers are the main route by which cattle are infected with TB, those same farmers are waiting until winter to test their cattle, giving additional time for infections to take root and spread to other animals.

The importance of public data

The study also emphasises the necessity of making all data on TB infections, badger culling and related matters public in order to facilitate independent scientific research:

"We would like to highlight the importance of pubic data. In order to predict and mitigate disease spread informed decisions are needed. These decisions need to be taken based on data analysis and predictive models calibrated with data. Our view is that making regional TB data available so that potential differences and underlying management decisions is a very good step forward.

"We argue that making publicly available the data regarding badger culling experiments as agents of TB infecting cattle will greatly facilitate their analysis and to informed decisions regarding TB control in GB."

Policy makers must listen!

Professor Alastair Macmillan, veterinary advisor at Humane Society International, commented: "This new paper provides extremely strong evidence of what many experts in veterinary disease control have known for many years - that it is crucial to test cattle as frequently as possible in order to control bovine TB.

"The Queen Mary researchers have shown without doubt that killing badgers will have little effect, whilst employing the policies of Wales and Scotland, where badgers are not culled, will continue to have a dramatic impact on reducing TB in cattle.

"Frequent cattle testing is particularly important as the sensitivity of currently available diagnostic tests is not very high, meaning that cattle incubating TB are not detected and are allowed to remain in the herd to infect others over the following months. These cattle are by far the most common reason why cattle herds suffer repeated TB breakdowns, not badgers.

"The government must heed this evidence and stop wasting time and resources on killing badgers to no effect. All efforts must instead be focused on far more frequent cattle testing and strict cattle movement control. How much more research and scientific evidence does this government need before it listens to the rational facts?"

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